Explain non-delivery notification

app/src/main/java/eu/faircode/email/Core.java

@@ -4914,7 +4914,7 @@ class Core {
 
             if (mdn && helper.isReport())
                 try {
-                    MessageHelper.Report r = parts.getReport();
+                    MessageHelper.Report r = parts.getReport(context);
                     boolean client_id = prefs.getBoolean("client_id", true);
                     String we = "dns;" + (client_id ? EmailService.getDefaultEhlo() : "example.com");
                     if (r != null && !we.equals(r.reporter)) {

app/src/main/java/eu/faircode/email/MessageHelper.java

@@ -4244,7 +4244,7 @@ public class MessageHelper {
                             HtmlHelper.formatPlainText(result) +
                             "</pre>";
                 } else if (h.isReport()) {
-                    Report report = new Report(h.contentType.getBaseType(), result);
+                    Report report = new Report(h.contentType.getBaseType(), result, context);
                     result = report.html;
 
                     StringBuilder w = new StringBuilder();
@@ -4286,7 +4286,7 @@ public class MessageHelper {
             return sb.toString();
         }
 
-        Report getReport() throws MessagingException, IOException {
+        Report getReport(Context context) throws MessagingException, IOException {
             for (PartHolder h : extra)
                 if (h.isReport()) {
                     String result;
@@ -4297,7 +4297,7 @@ public class MessageHelper {
                         result = Helper.readStream((InputStream) content);
                     else
                         result = content.toString();
-                    return new Report(h.contentType.getBaseType(), result);
+                    return new Report(h.contentType.getBaseType(), result, context);
                 }
             return null;
         }
@@ -6062,7 +6062,7 @@ public class MessageHelper {
         String feedback;
         String html;
 
-        Report(String type, String content) {
+        Report(String type, String content, Context context) {
             this.type = type;
             StringBuilder report = new StringBuilder();
             report.append("<hr><div style=\"font-family: monospace; font-size: small;\">");
@@ -6138,7 +6138,14 @@ public class MessageHelper {
                 Log.e(ex);
                 report.append(TextUtils.htmlEncode(new ThrowableWrapper(ex).toSafeString()));
             }
+
             report.append("</div>");
+
+            if (isDeliveryStatus() && !isDelivered())
+                report.append("<br><div style=\"font-size: small; font-style: italic;\">")
+                        .append(TextUtils.htmlEncode(context.getString(R.string.title_report_remark)))
+                        .append("</div>");
+
             this.html = report.toString();
         }
 

app/src/main/res/values/strings.xml

@@ -1681,6 +1681,7 @@
     <string name="title_attachment_saved">Attachment saved</string>
     <string name="title_attachments_saved">Attachments saved</string>
     <string name="title_attachments_incomplete">Some attachments or images were not downloaded and could not be added</string>
+    <string name="title_report_remark">This report, a specially formatted email, was sent by an email server because a sent message could not be delivered to the recipient, for example because the mailbox is full, or because the email address does not exist (anymore).</string>
     <string name="title_ask_save">Save changes?</string>
     <string name="title_ask_delete">Delete message permanently?</string>
     <string name="title_ask_delete_accept">I understand that permanently deleting messages is irreversible</string>